This invention relates to semiconductor III-V alloy compounds, and more specifically to DBR VCSELs.
The importance of semiconductor emitters and detectors is rapidly increasing along with progress in the opto-electronic field, such as optical fiber communication, optical data processing, storage and solid state laser pumping.
VCSELs emitting at xcex=0.98 xcexcm are very attractive devices for optical communication systems and other applications because of their extremely low power requirement, high efficiency, circular beam output, and two dimensional scalability. Virtually every device reported to date contains Al in the AlGaAs distributed Bragg reflector (DBR) mirror stacks that serve as facets for the vertical optical cavity. However recently Al-free 808 nm and 980 nm laser diodes in edge-emitting lasers appear superior over those containing Al. Thus it would appear desirable to include an oxide-confined GaInAs/GaAs active region in a VCSEL through a simple selective regrowth step that will give an oxide-confined VCSELs without the use of Al.
Because they emit normal to the surface and are readily fabricated into two-dimensional arrays, vertical cavity surface emitting lasers (VCSELs) are ideally suited as light sources for optical fiber communication, digital printing and scanning, and optical disk storage. Other advantages VCSELs possess over edge-emitting lasers are ease of fabrication and testing, circular beam output, and low-bias, low-threshold operation.
Another major difference in a VCSEL is its microcavity structure. VCSELs range in size from as little as 1 xcexcm up to 100 xcexcm in diameter, much smaller than their edge-emitting counterparts, hence its output beam will be much smaller as well. How the optical cavity is defined is crucial to its performance. One early method used an air post where the as-grown structure was etched into cylindrical pillars, and the size of the pillar determined the lateral size of the cavity. Another method for the purpose of transverse carrier confinement is to ion implant the outlying area to force the carriers to flow though the active region. The lateral cavity size was determined by the window of the unimplaned area.
Unfortunately, these methods suffer from the problem of current spreading into the surrounding region, which raises the threshold current and degrades laser performance. Also, for air post structures sidewall nonradiative recombination is excessive, and for ion implanted structures, implantation damage is present. Both of these occurrences also degrade performance. However, recently a relatively new technique involving oxidation of the outlying area was introduced to laterally define the optical cavity and improved VCSEL performance.
It was well known that semiconductor materials containing Al, such as AlGaAs, are chemically unstable in a normal atmospheric environment and hydrolyze over time to form the stable native oxide AlxOy. This was always an unwanted effect and had serious consequences for the lifetimes of lasers containing Al. But there has been developed a controlled method to oxidize AlGaAs into AlxOy at high temperatures with water vapor in an N2 environment. This process of wet oxidation was then applied to transverse current confinement in VCSELs.
Superb results have been obtained from VCSELs with oxidized apertures, including record wall-plug efficiency and threshold current. However, the lifetime and reliability of these lasers is questionable since they do contain Al, which oxidize easily and degrade much more rapidly than Al-free lasers. This has already been demonstrated in edge-emitting lasers.
An object, therefore, of the invention is a Vertical Cavity Surface Emitting Laser for use in optical communication and other fields.
A further object of the subject invention is a Al-free VCSEL with InGaP/GaAs DBRs (Distributed Bragg Reflector).
A still further object of the subject invention is an oxide confined Al-free VCSEL.
These and other objects are attained by the subject invention wherein an Al-free VCSEL is grown by MOCVD procedure by growing GaInP/GaAs as a conventional distributed Bragg reflector (DBR). By the subject invention, 36 or less periods are formed as the active layer and achieve a reflectivity of 0.99, at 0.98 xcexcm. The DBRs are composed of repeating layers of a 69 nm period of GaAs and a 76 nm period of InGaP to form a superlattice as quarter wave thickness stacks. The measured and calculated reflectivity is shown in FIG. 1; there is very good agreement between the two. The high resolution x-ray diffraction spectrum, pictured in FIG. 2, reveals over 25 orders of satellite peaks, indicating the excellent crystalline and interfacial quality of this structure. For localized epitaxy, after the lower layer of n-type DBR is deposited by MOCVD, a lift-off procedure opens up windows in an evaporated layer of SiO2. The active region and upper p-type DBR is then deposited by MOCVD.